Japanese Patent Application No. 2000-270443, filed Sep. 6, 2000, is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a semiconductor device having an amplifier whose input resistance and feedback resistance are both variable.
2. Description of Related Art
There is a semiconductor device which has variable resistors respectively connected to the input terminal and feedback input terminal of an amplifier incorporated in the semiconductor device. A variable resistor has a plurality of connection points in a resistor layer having a predetermined length and changes its resistance as one of the connection points is selected.
An amplifier of this kind has a problem such that when the input resistance and feedback input resistance are changed, the output voltage of the amplifier varies more than the allowable value, thus hindering circuit functions. The cause for this hindrance is a variation in the resistances of the variable resistors that are connected to the input terminal and feedback input terminal of the amplifier.
Normally, a variable resistor of this type is formed by placing a resistor layer in a region which does not interfere with the formation of transistors. In addition, the resistor that is connected to the input terminal of the amplifier and the resistor that is connected to the feedback input terminal of the amplifier are formed apart from each other.
An objective of the present invention is to provide a semiconductor device in which a voltage dividing ratio or a ratio of the length from one end of a first resistor to be connected to an input terminal of an amplifier to each connection point to the entire length of the resistor, and another voltage dividing ratio or a ratio of the length from one end of a second resistor to be connected to a feedback input terminal of the amplifier to each connection point to the entire length of the resistor can be set as designed.
According to the present invention, there is provided a semiconductor device comprising:
an amplifier which has an input terminal, a feedback input terminal and an output terminal;
a first resistor which has a plurality of first connection points to be selectively connected to the input terminal of the amplifier and has both ends to which a voltage is applied;
a second resistor which has one end to be connected to the output terminal of the amplifier and has a plurality of second connection points to be selectively connected to the feedback input terminal of the amplifier; and
a connection-switching circuit which connects one of the first connection points to the input terminal and one of the second connection points to the feedback input terminal in such a manner that a voltage at the output terminal of the amplifier becomes constant,
wherein each of the first and second resistors is formed by connecting a plurality of reference resistors of at least one reference length by means of interconnect lines.